xe2x80x9cNot Applicablexe2x80x9d
This invention relates to an apparatus and a method for building precise 3D components and structures by a material addition process called laser consolidation, more particularly an arrangement for the vertical delivery of metallic powder, or wire into a precisely formed melt pool created in a substrate by laser beams having a specific angular orientation relative to the substrate.
Rapid Prototyping (RP) is a related technique based on layered manufacturing where a part is built as a series of horizontal layers, each one being formed individually and bonded to the preceding layer. Various processes have been used differing in the way each layer is formed and the raw materials used but the underlying methodology is essentially the same in each case.
Stereolithography (SLA) and Selected Laser Sintering (SLS) are the two most common rapid prototyping processes. In both cases, a three dimensional CAD model of a part is generated and sliced into horizontal layers. The sliced files are used for tool path generation to make a solid part layer by layer. The thickness of each slice is controlled and is determined by the degree of accuracy required and the capability of the system, viz-a-viz the maximum thickness that can be cured or sintered by the specific process.
The SLA process uses a photosensitive monomer, which is cured layer by layer using an ultraviolet laser resulting in a cured polymer part. In the SLS process a carbon dioxide laser of appropriate power is used to scan across the surface of a bed of a powdered thermoplastic material, sintering the powder into the shape of the required cross-section. A major limitation of the SLS process is its inflexibility in the selection of metals that can be used. To generate metallic parts, thermoplastic coated metal powders are used to create a xe2x80x9cgreen shapexe2x80x9d of the component. The thermoplastic plastic is removed in a xe2x80x9cburn-offxe2x80x9d step and replaced by infiltrating a lower melting point metal.
In order to produce dense three dimensional metal/alloy parts, Los Alamos National Laboratory in the U.S. developed a process called xe2x80x9cDirected Light Fabrication of Complex Metal Partsxe2x80x9d (1994 ICALEO conference). In this process a coaxial powder delivery nozzle is used with a normal laser incident angle. The focused laser beam enters a chamber along the vertical axis of the nozzle that also delivers metal powder to the focal zone. The deposition is done on a base plate, which is removed after the part is built. The powders used for part build-up are 316 stainless steel, pure tungsten, nickel aluminide and molybdenum disilicide.
In a paper presented at a xe2x80x9cRapid Prototyping and Manufacturing xe2x80x9c96xe2x80x9d conference (SME, Michigan, Apr. 23-25, 1996) Dave Keicher of Sandia National Laboratories dealt with xe2x80x9cLaser Engineered Net Shaping (LENS) for Additive Component Processingxe2x80x9d. This process uses a Nd:YAG laser and a special nozzle arrangement for powder delivery. Four streams of powder are fed into a melt pool which is created and sustained by a central laser beam. It is pointed out that this arrangement avoids the situation in off-axis single side feed powder delivery system where there is a strong directional dependence. The symmetrical (quasi coaxial) arrangement permits uniform cladding independent of direction.
A rapid prototyping technique has also been used by Prof. W. Steen (1994 ICALEO conference paper). A machining pass is added after each build-up pass, and a high power carbon dioxide laser ( greater than 2 kw) is used. Optics for the beam delivery system are incorporated on an automatic tool changing system. The process requires that after each laser build-up pass, the metal layer is machined back to required dimensions, necessary because of a lack of control on the laser build-up. It was also found that a change in cladding direction has a significant influence on the shape and quality of the build-up. Good quality clad with a regular shaped bead was obtained parallel to the flow direction but as the angle to the flow direction increased the quality deteriorated until clad perpendicular to the flow was of poor quality. Machining is used to remove the imperfections in shape and size of each built up layer arising from the change in the clad direction. As side nozzle powder delivery builds unevenly in various directions in the xy-plane, the additional required step of machining after each deposition pass makes the process cumbersome and expensive. As the control on the build-up process is poor, most of the material is removed to maintain the geometry creating unnecessary waste of expensive material.
It is evident from the above that in building up metal parts using a carbon dioxide or Nd:YAG laser and metallic powder, single nozzle side delivery always involves a directional dependence, and is either abandoned in favor of coaxial powder delivery or machining is employed after every pass to maintain dimensions. The trend is to use a coaxial powder delivery to obtain equal layer build-up in all directions. In addition it is apparent that the incident laser beam is always normal to the surface of the base plate.
Several nozzle designs for coaxial powder feeding during laser cladding have been disclosed, for example: U.S. pat. No. 4,724,299 (Hammeke, Feb. 9, 1988); U.S. Pat. No. 5,418,350 (Freneaux, May 23, 1995); U.S. Pat. No. 5,477,026 (Buongiorno, Dec. 19, 1995) and U.S. Pat. No. 5,111,021 (Jolys, May 5, 1992).
U.S. Pat. No. 5,731,046 to Mistry (Mar. 24,1998) discloses a technique for fabricating diamond and diamond-like coatings on a substrate. Mistry also discloses that complex shapes can be fabricated as coating structures on the surface of the substrate. Mistry discloses using a plurality of lasers each having different and specific temporal and spectral characteristics to perform the following functions: one laser to ablate the constituent element, a second to initiate chemical reaction, and a third to provide overall thermal balance. Mistry discloses that shaped coatings can be made on the surface of the substrate by the relative movement of the laser system and the substrate. Ministry does not teach the importance of the critical angle of the lasers relative to the powder feed nozzle, the symmetrical arrangement of the laser beams relative to the material feed system nor the control over and the shape of the melt pool required to make precise structures and components with smooth walls.
The inventors"" U.S. Pat. No. 5,855,149 (Canadian application 2,242,082 published Dec. 30,1999) teaches a method of producing a sharpened edge on a cutting die by having a laser beam or beams impinge on a base surface at an angle to the normal of between 5xc2x0 and 45xc2x0 to fuse successive thin layers forming a metal ridge to the cutting edge. The inventors"" Canadian application No. 2,215,940 published Mar. 23,1998 discloses an apparatus and method for material disposition on a surface using a laser beam or beams impinging on the surface at an angle to the normal of between 5xc2x0 and 45xc2x0.
Generally laser based material addition processes rely on focusing a laser beam to create a small molten zone in a suitable starting material (substrate). New material, usually in powder form, is added and melted to increase the volume of the molten zone. When the laser is shut off, or moved to a new location, the molten material rapidly cools and solidifies. When the process is sustained by moving the laser and material addition system across the substrate, at a controlled speed, it is possible to make a uniform ridge. The ridge can take on geometric forms when the laser and powder feed systems are moved across the substrate by following a predetermined path as described by a computer numerically controlled system. By repeating the operation using the original ridge as a new substrate, eventually after subsequent layers are added, a walled structure is formed.
All of the processes reported, can be described as near net shape. For example, Sandia National Laboratories, using their Laser Engineered Net Shape (LENS(trademark)) process, can produce parts with complex shapes having surface finishes that resembles a fine sand casting and having dimensional tolerances at best of +/xe2x88x92100 microns. To obtain better dimensional control and surface finishes requires secondary operations.
The arrangements commonly used in the prior art as illustrated in FIG. 1 of the drawings have a central laser source 20 with powder 21 entering symmetrically from the sides around the circumference. In this way the relationship to the pool remains the same regardless of the wall path. The arrangement is symmetrical but powder entering from the sides causes thermal and viscosity gradients 22, leading to incomplete melting where the wall surfaces are forming. Line 23 indicates the boundary of the molten zone. Unmelted or partially melted particles of powder 24 tend to stick on the surface as the wall is cooling. Attempting to correct the situation by adding more energy is not successful because the surface where the energy enters starts to evaporate causing a plasma to form which absorbs the incoming laser energy. The mass of xe2x80x9csoupyxe2x80x9d unmelted material in the vicinity of sides of the wall tends to slump outside the dimensions of the pool. Subsequent passes, or layers, applied in this slumped condition result in a wall where each layer has a convex curved surface 25. These curves at the surfaces of the layers produce variations in the wall thickness 26. The resultant wall has the appearance similar to 27 shown in figure 1a. 
The practice of making precise structures in the prior art is to form a rough shape then use a material removal operation such as machining to create the final shape and surface finish. The present application describes a methodology and apparatus for making precise structures, for example, in the form of shells, in one operation.
When a focused laser is used to rapidly melt a zone in a substrate, and the zone is cooled quickly, the surface of the solidified zone is smooth. When the melting takes place in a non-oxidizing, dust and vibration free environment, and the molten zone is maintained close to the flow temperature of the substrate material, submicron finishes can be obtained on the solidified surface. If the melting process is controlled it is possible to get high quality surface finishes.
When material is added and melted into the melt pool, to increase its volume, it is more difficult to maintain a smooth finish. The problems with existing state of the art near net shape processes that feed powder into the pool from the sides stem from the thermal, and hence viscosity gradients, created in the pool and from powder particles sticking to the side walls as the pool solidifies.
In the invention:
Laser energy enters the molten pool at an angle of about 30 degrees to the vertical and symmetrically around the pool in the form of an annulus.
Powder is injected vertically at the top dead center of the melt pool through a fine nozzle.
The advantages are:
In forming the pool of molten material, energy enters symmetrically around the pool allowing the temperature to rise uniformly and rapidly avoiding local evaporation or the creation of serious thermal gradients within the pool.
Surface tension is maintained uniformly around the pool and hence results in a pool with a surface that is close to hemispherical in shape.
The temperature of the pool in the regions where the walls will form is uniform from side to side and is controlled above the melt temperature so that all the powder is completely melted. Thus the walls formed on cooling have a precise width and the surfaces are smooth. There are no visible or metallurgical discontinuities to show that the structure has been formed in a series of passes.
Directing the powder into the pool at the top ensures a high capture rate of powder and any stray particles are directed through the incoming beam and away from the solidifying wall surfaces.
The symmetry of the total system permits the substrate to be moved in any direction relative to the laser powder feed arrangement without changing the thermal balance within the melt pool.
The apparatus of this invention meets the criteria for making precise walls. However, in practice there may be a need to make minor adjustments in wall thickness. The fixed focus rigid 360 degree focusing mirror precludes any adjustment.
Two other symmetrical laser variations permitting adjustment are disclosed which produce acceptable results.